1. Field
Embodiments of the invention relate to a treatment composition containing a mixture of a cross-linking agent and an anti-hydrogen-bonding agent for making acquisition fluff pulp with low centrifuge retention capacity in sheet form. Embodiments of the present invention also relate to a process for making the acquisition fluff pulp in sheet form. The fluff pulp can be characterized as having improved resiliency, bulk, and acquisition rate, which makes it suitable for use in an acquisition layer of absorbent articles intended for body fluid management.
2. Description of Related Art
Absorbent articles intended for personal care, such as adult incontinent pads, feminine care products, and infant diapers typically are comprised of at least a top sheet, a back sheet, an absorbent core positioned between the top sheet and back sheet, and an optional acquisition/distribution layer positioned between the top sheet and the absorbent core. An acquisition/distribution layer usually is incorporated in the absorbent articles to provide better distribution of liquid, increased rate of liquid absorption, reduced gel blocking, and improved surface dryness. The acquisition/distribution layer may be comprised of, for example, synthetic fibers, a composite of cellulosic fibers and synthetic fibers, or cross-linked cellulosic fibers. Cross-linked cellulosic fiber is preferred because it is abundant, it is biodegradable, and it is relatively inexpensive.
Cross-linked cellulosic fibers and processes for making them have been described in the literature for many years (see, for example, G. C. Tesoro, Cross-Linking of Cellulosics, in Vol. II of Handbook of Fiber Science and Technology, pp. 1–46 (M. Lewin and S. B. Sello eds., Mercel Dekker, New York, 1983)). The cross-linked cellulosic fibers typically are prepared by reacting cellulose with polyfunctional agents that are capable of covalently bonding to at least two hydroxyl groups of the anhydroglucose repeat unit of cellulose in neighboring chains simultaneously.
Cellulosic fibers typically are cross-linked in fluff form. Processes for making cross-linked fiber in fluff form comprise dipping swollen or non-swollen fiber in an aqueous solution of cross-linking agent and a catalyst. The fiber so treated, then is usually cross-linked by heating it at elevated temperature in the swollen state, as described in U.S. Pat. No. 3,241,553, or in the collapsed state after defiberizing it, as described in U.S. Pat. No. 3,224,926, and European Patent No. 0,427,361 B1, the disclosures of each of which are incorporated by reference herein in their entirety.
Cross-linking of fibers is believed to improve the physical and the chemical properties of fibers in many ways, such as improving the fibers' wet and dry resiliency, increasing fluid absorbency, reducing wrinkling, and improving shrinkage resistance. However, cross-linked cellulosic fibers have not been widely adopted in absorbent products, seemingly because of the difficulty of making cross-linked cellulosic fibers in sheet form. More specifically, it has been found that cross-linked fiber in the sheet form tends to create substantial problems when defiberized. These problems include severe fiber breakage and increased amounts of knots and nits (hard fiber clumps). Furthermore, such cross-linked fibers demonstrate an unpleasant odor and low fiber brightness. These problems render the cross-linked fiber unsuitable for applications in absorbent articles intended for body waste managements.
The difficulties associated with defiberizing fibers cross-linked in sheet form were attributed to the considerable amount of lignin and hemicellulose that remains in the fiber after the pulping and bleaching processes. Large portions of these residuals are distributed on the surfaces of the fibers. These residuals under the heating conditions of the cross-linking reaction combine with the cross-linking agents to form thermosetting adhesives. As a result, strong bonding forms between adjacent fibers so that it is very difficult to separate them under any conditions without considerable fiber breakage. Because the cross-linked fibers tend to be brittle, the fibers themselves will often break, generating fibers with high contents of knots, nits and fines.
Efforts to make cross-linked fibers in sheet form have met with limited success. Some attempts have involved minimizing the contact between fibers in the sheet then cross-link the fibers. For example, Graef et al. in U.S. Pat. No. 5,399,240, the disclosure of which is incorporated herein by reference in its entirety, discloses a method of making a sheet of fibers containing a de-bonding agent, followed by treating the sheet while in the wet state with a mixture of a cross-linking agent and a catalyst. The de-bonder used is composed of a fatty chain and quaternary ammonium group. The fatty chain tends to interfere with the hydrogen bonding between fibers during sheet formation. This reduction in hydrogen bonding leads to a softer and partially bonded sheet of fluff pulp. While in sheet form, the fiber is then cured at elevated temperatures to produce cross-linked fibers with a relatively low content of knots and nits. Unfortunately, the long hydrophobic alkane chain tends to adversely affect the absorbency and wettability of the fiber, rendering it unsuitable for applications such as in absorbent articles, where a high rate of absorbency and fast acquisition are essential.
In U.S. Pat. No. 3,434,918, Bernardin et al. discloses a method of treating fibers in sheet form with a cross-linking agent and a catalyst. The treated sheet then is wet-aged to render the cross-linking agent non-extractable. The wet-aged fibers are re-dispersed before curing, mixed with untreated fibers, sheeted, and then cured. The mixture of cross-linked fibers and untreated fibers are potentially useful for making products such as filter media, tissues, and toweling where high bulk and good water absorbency are desired without excessive stiffness in the product. Unfortunately, the presence of untreated fibers make the produced fibers have low performance as an acquisition/distribution layer in hygiene products such as diapers.
Other documents describing methods of treating fiber in sheet form include, for example, U.S. Pat. Nos. 4,204,054; 3,844,880; and 3,700,549 (the disclosures of each of which are incorporated by reference herein in their entirety). However, the above-described approaches complicate the process of cross-linking fiber in sheet form, and render the process time consuming, and costly. In addition to that, these processes result in cross-linked fibers with a substantial decrease in fiber performance, and a substantial increase in cost.
In order to avoid such problems, Chatterjee, et al., showed in U.S. Pat. No. 3,932,209 (the disclosure of which is incorporated herein by reference in its entirety) that mercerized fiber having low contents of hemicellulose and lignin can be cross-linked in sheet form without substantial formation of knots and nits. Unfortunately, the use of mercerized fiber to produce cross-linked fiber in sheet form is relatively expensive.
In previous work, (e.g., U.S. patent application Ser. No. 10/683,164 entitled “Materials Useful In Making Cellulosic Acquisition Fibers In Sheet Form” filed Oct. 10, 2003, the disclosure of which is incorporated herein by reference in its entirety) it was shown that conventional fibers can be successfully cross-linked in sheet form using modified cross-linking agents. The modified cross-linking agent acts as a cross-linking agent and as a wedge that lowers the inter-fiber bonding and increase fiber bulkiness. This way, the formation of knots and nits as well as fines during fiber cross-linking were minimized. The produced cross-linked fibers showed similar or better performance characteristics than conventional individualized cross-linked cellulose fibers.
The description herein of certain advantages and disadvantages of known cellulosic fibers, treatment compositions, and methods of their preparation, is not intended to limit the scope of the embodiments. Indeed, the embodiments may include some or all of the methods, fibers and compositions described above without suffering from the same disadvantages.